1. Field of the Invention
The present invention relates to an organometallic complex. In particular, the present invention relates to an organometallic complex that is capable of converting a triplet excited state into luminescence. Further, the present invention relates to a light-emitting element, a light-emitting device, and an electronic device which include the organometallic complex.
2. Description of the Related Art
Organic compounds are brought into an excited state by absorbing light. By going through this excited state, various reactions (photochemical reactions) are caused in some cases, or luminescence is produced in some cases. Therefore, various applications of the organic compounds have been being made.
As one example of the photochemical reactions, a reaction (oxygen addition) of singlet oxygen with an unsaturated organic molecule is known (for example, see Non-Patent Document 1). Since the ground state of an oxygen molecule is a triplet state, oxygen in a singlet state (singlet oxygen) is not generated by a direct photoexcitation. However, in the presence of another triplet excited molecule, singlet oxygen is generated to achieve an oxygen addition reaction. In this case, a compound that can become the triplet excited molecule is referred to as a photosensitizer.
As described above, in order to generate singlet oxygen, a photosensitizer that can become a triplet excited molecule by photoexcitation is necessary. However, since the ground state of an ordinary organic compound is a singlet state, photoexcitation to a triplet excited state is a forbidden transition, and a triplet excited molecule is unlikely to be generated. Therefore, as such a photosensitizer, a compound in which intersystem crossing from the singlet excited state to the triplet excited state easily occurs (or a compound in which the forbidden transition of photoexcitation directly to the triplet excited state is allowed) is required. In other words, such a compound can be used as a photosensitizer, and is useful.
Further, such a compound often emits phosphorescence. Phosphorescence refers to luminescence generated by transition between different energies in multiplicity. In an ordinary organic compound, phosphorescence refers to luminescence generated in returning from the triplet excited state to the singlet ground state (in contrast, luminescence in returning from a singlet excited state to a singlet ground state is referred to as fluorescence). Application fields of a compound capable of emitting phosphorescence, that is, a compound capable of converting an energy difference between a triplet excited state and a ground state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light-emitting element using an organic compound as a light-emitting substance.
This light-emitting element has a simple structure in which a light-emitting layer including an organic compound that is a light-emitting substance is provided between electrodes. This light-emitting element has attracted attention as a next-generation flat panel display element in terms of characteristics such as being thin and light in weight, high-speed response, and direct current low voltage driving. In addition, a display device using this light-emitting element is superior in contrast, image quality, and wide viewing angle.
The light-emitting element including an organic compound as a light-emitting substance has a mechanism of light emission that is carrier injection: voltage is applied between electrodes where a light-emitting layer is interposed, electrons and holes injected from the electrodes are recombined to make the light-emitting substance excited, and then light is emitted in returning from the excited state to the ground state. As in the case of photoexcitation described above, types of the excited state include a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3.
As for a compound capable of converting a singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound), luminescence from a triplet excited state (phosphorescence) is not observed but only luminescence from a singlet excited state (fluorescence) is observed at a room temperature. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, in the case of a light-emitting element including the phosphorescent compound described above, the internal quantum efficiency thereof can be improved to 75% to 100% in theory; namely, the emission efficiency thereof can be 3 to 4 times as much as that of a light-emitting element including a fluorescent compound. Therefore, the light-emitting element including a phosphorescent compound has been actively developed in recent years in order to achieve a high efficient light-emitting element (for example, see Non-Patent Document 2). An organometallic complex that contains iridium or the like as a central metal is particularly has attracted attention as a phosphorescent compound because of its high phosphorescence quantum efficiency.